Referring to FIGS. 1 and 2 for a circuit diagram and a waveform diagram of a traditional post regulation control circuit respectively, the post regulation control circuit detects a sync signal (trailing edge trigger) produced by a trailing edge of a voltage waveform of a secondary coil of a transformer T1 and uses the sync signal as a reset signal for resetting a ramp generator. For each reset signal, the ramp voltage is discharged to zero and then reset to a maximum electric potential to produce a stable continuous ramp voltage. Further, the voltage of an input signal is divided by an impedance Z1 and a resistor R2 and outputted through an error amplifier, and then an inverting amplifier (−GAIN amplifier) produces a feedback voltage directly proportional to the output voltage. After the feedback voltage is compared with the ramp voltage, a power conduction signal (HS-DRV) corresponding to a stable output voltage is produced. After the power conduction signal (HS-DRV) is cut off, a flywheel conduction signal (LS-DRV) in an inverted phase will be produced. The power conduction signal (HS-DRV) drives a power conduction power switch SW1 to be conducted electrically, and the flywheel conduction signal (LS-DRV) drives a flywheel conduction power switch SW2 to be conducted electrically, and the delay control circuit produces an output delay to prevent a short circuit when the two power switches are conducted electrically at the same time. Referring to FIG. 1 for a circuit diagram of a traditional post regulation control circuit, a ramp generator in a discontinuous conduction mode (DCM) produces unstable ramp voltages easily which may adversely affect the stability of the output voltage.
Therefore, finding a way of designing a post regulation control circuit of a power supply that produces a stable output voltage for a power supply in a discontinuous conduction mode (DCM) is a subject of the present invention.